While it is possible to put two routes in place with the secondary having a higher metric, this may not always work. In the event of a failure that does not bring down the physical interface on the router the primary route is not removed. There is also the situation where the primary interface takes too long to change status. The way around these limitations is simple; IP SLA

Here's how to do it

ip sla 1   < The number 1 here is arbitrary, used only to identify this sla. It is otherwise knows as the operation number>

icmp-echo 4.2.2.2  < 4.2.2.2 is a DNS server that responds to pings out on the internet>

timeout 500  < This is how long to wait for a response from the ping>

frequency 3 < This is the repeat rate for the SLA>

ip sla schedule 1 start-time now life forever < This command says  "start SLA 1 now and keep it running forever>

track 1 rtr 1 reachability   < This comand creates the track object "1" and monitors the SLA 1>

now for the routing, we need to change the default route and associate it with the tracker

no ip route 0.0.0.0 0.0.0.0 1.1.1.1

and then put it back with the tracking

ip route 0.0.0.0 0.0.0.0 1.1.1.1 track 1 

Then we need to add our secondary route

ip route 0.0.0.0 0.0.0.0 1.1.1.2 10 <we set this route with a higher metric than the tracked route>

Now when the ping to 4.2.2.2 fails the primary route is removed and the secondary route with the higher metric becomes the default. The route will be reinstated when the connectivity is restored.

With the 12.4 and higher releases the commands have changed slightly but the "?" is your friend. If I receive requests for the syntax I will post it as well, but it is pretty easy to convert.

Here is the reference to the Cisco IP SLA documentation

http://www.cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a0080441845.html